I honestly don't know why God didn't heal your son. Some day in heaven, we'll know the answer to questions like this -- but until then, all we can do is trust God and look to Him for the strength we need to deal with life's hard places.
I do know this: God loves your son, just as He loves you. And one of the signs of His love for him is that God has given him to you. Think of it: What if your son had been born into a family that rejected him or didn't want him? But he wasn't. Instead, God gave him to you -- a family that loves him and will do their best to meet his needs.
Over the years, my wife and I were privileged to know many families who had a child with special mental or physical needs. Like you, they loved their children -- and without exception, over time they came to see them as special gifts from God.
One of those friends, Dale Evans (wife of cowboy actor Roy Rogers), wrote a book about their experience entitled "Angel Unaware," saying that God had blessed them so much through their child that it was almost as if an angel had come to live with them. The title was a reference to Hebrews 13:2: "Be not forgetful to entertain strangers: for thereby some have entertained angels unawares" (KJV). May this be your experience, as well.